The present invention relates generally to the templated growth of porous or non-porous castings, and more specifically to the formation of porous or non-porous castings via directional solidification within a honeycomb substrate.
Porous substrates can be used in a wide variety of applications including catalysis, filtration, membranes, etc. And while numerous processing techniques such as extrusion and foaming have been developed to produce porous and other cellular materials, it remains difficult during processing to precisely control the size, shape, directionality, isotropy (or anisotropy), connectivity and tortuosity of the porosity and/or the cellular structure. An additional challenge in forming cellular materials relates to the ubiquitous use of pore forming agents and the difficulties associated with removing them from the final product.
Non-porous substrates comprising crystalline metals (e.g., Cu) and semiconductors (e.g., Si, GaAs, etc.) can be used in both semiconductor and non-semiconductor applications. For example, ingots of polycrystalline silicon can be prepared for photovoltaic applications via directional solidification in quartz crucibles as an alternative to Czochralski (CZ) growth. In addition to semiconductor materials, directional solidification can also be used to form metallic and ceramic substrates. As used herein, a substrate is a material upon which a process is conducted, or upon which a device or structure is constructed.
Directional solidification relates generally to a series of measures applied to control a liquid-to-solid phase transformation and the attendant formation of a solidified or cast body. Thus, the cast body or cast structure described herein is a solid shape. Directional solidification involves passing a liquid or molten feedstock through a temperature gradient in order to cause freezing (solidification) of the feedstock.
The liquid feedstock can comprise a single-component liquid such as molten silicon or a multi-component liquid such as molten alloys (e.g., silicon-germanium alloys). In turn, multi-component liquid feedstocks can comprise either a single phase (e.g., molten Si—Ge alloys or water-PVA solutions) or multiple phases. An example of a multi-phase liquid feedstock is a dispersion of gas bubbles, immiscible liquid and/or solid particles in one or more liquids. A dispersion can be an emulsion or a colloid.
Directional solidification can be used to form porous or non-porous materials, including metals, semiconductors, ceramics, polymers or composites thereof. Microstructured, cellular materials, for example, can be cast from a multi-component feedstock that comprises a dispersion (i.e., slurry) of particles in a liquid. The process involves solidifying the liquid feedstock, removing the solidified (previously liquid) phase, and optionally densifying the resulting structure. In the case of unidirectional freezing, a porous body can be formed having unidirectional channels (i.e., linear porosity) where the channels are formed from the volume previously occupied by the solidified phase. In turn, non-porous castings can be made from single component or multi-component liquid feedstocks such as, for example, molten silicon or molten metal alloys. Directional solidification can be used as a near net shape forming route.
In both single- and multi-component liquid feedstock systems, in order to control the microstructure and hence the resulting properties of the solidified material, it is important during solidification to maintain a spatially-uniform solidification front (solid-liquid interface). In addition, in multi-component systems it is important to maintain a laterally uniform distribution of particles and/or solute along the solid-liquid interface. These conditions are difficult to achieve in practice due to the presence of density gradient-driven convection in the liquid phase.
Convection in directional solidification results from inevitable thermal gradients within the system. Even in the example of vertically stable density stratification, the radial temperature gradients that are present in the liquid can produce convention currents. The convection currents can generate severe solute segregation as well as non-uniform (i.e., macroscopically-curved) solidification fronts. The solute segregation, in turn, can lead to concentration gradient-driven convection that may assist or oppose thermally driven convection. In the example of particulate-laden feedstocks, the convection currents can sweep the particles along the solid-liquid interface, which can lead to a highly non-uniform particle distribution. This problem will generally become more pronounced as the effective dimension of the liquid increases.
When producing porous castings via directional solidification, other issues that can be encountered, particularly with larger volume castings, are the loss of pore continuity or connectivity during freezing due to nucleation and growth of non-parallel grains and the inadequacy of green body strength in pre-densified samples.
In view of the foregoing, it would be advantageous to develop a method for preparing porous or non-porous castings via directional solidification that reduces thermal and/or solutal convection induced non-homogeneity along the solid-liquid interface, reduces the lack of green body strength in scaled-up samples, and reduces the loss of axial connectivity and the increase in tortuosity in porous castings due to the nucleation, growth and impingement of non-parallel grains.
According to one embodiment, a method of forming a templated casting comprises incorporating a liquid feedstock into the channels of a honeycomb substrate to form a feedstock-laden substrate, and directionally solidifying the liquid feedstock within the channels. The casting can comprise a porous or a non-porous structure.
A porous casting can be formed by solidifying a multi-component feedstock. According to one non-limiting example, the formation of a porous casting comprises incorporating a liquid dispersion into channels of a honeycomb substrate to form a dispersion-laden substrate, the dispersion comprising particles dispersed in a liquid, moving the dispersion-laden substrate into a cooled zone of a solidification apparatus in order to directionally solidify the liquid within the channels, and removing the solidified liquid from within the channels to form a structure that comprises a porous body of the particles within the channels. Optionally, the particles, which can comprise one or more of metallic, semiconducting, ceramic and polymeric particles, can be sintered or impregnated to densify the cast structure. A non-porous casting can be formed by solidifying a single-component or a multi-component feedstock by omitting the act of removing the solidified phase.
In a further embodiment, a templated casting includes a honeycomb substrate having a plurality of channels, and a directionally-ordered cast structure incorporated within the channels. The honeycomb substrate itself can comprise a metallic, semiconducting, ceramic or polymeric material, or mixtures or composites thereof. By way of example, a honeycomb substrate can be formed from compounds such as plaster of Paris (e.g., CaSO4.0.5H2O) or elements such as sulfur. Optionally, the honeycomb substrate can be removed to yield a plurality of directionally-ordered castings of metallic, semiconducting, ceramic or polymeric material, or mixtures or composites thereof.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention.